She was at ground zero just after September 11th, supporting workers who combed through the smoking debris in search of human remains.

"This is a very proud moment; it's full circle," she said, glancing across the 16-acre site where the uncompleted One World Trade Center in the northwest corner is already New York's tallest structure.

In the southeast corner facing the 9/11 memorial, the 72-story tower that was topped off is to open for business in 2013 - the first occupied high-rise at the new trade centre site since the attacks.

The 1.8-million-square-foot skyscraper, designed by Pritzker Prize-winning architect Fumihiko Maki, will primarily house commercial offices.

A third of the office space will be set aside for the headquarters of the Port Authority of New York and New Jersey, which owns the site.

An atrium will house shops and restaurants.

At the moment, it is the second tallest skyscraper on the rebuilt World Trade Center site after One World Trade Center, although two others towers eventually will surpass the height of 4 World Trade Center.

More than 100 construction workers signed their names to the white-painted steel, with elected officials and developer Larry Silverstein looking on.

"Everybody's put their blood, sweat and tears into this," said John Rzeznik, a project manager at the site.

Minutes earlier, in the half-completed, marble-graced entrance, Winans opened the ceremony by singing the national anthem. Listening to it, some construction workers were moved to hold their hardhats over their hearts.

Silverstein told those assembled that his goal as a developer was "to give New Yorkers back the city terrorists tried to take away."

After years of funding and planning disagreements that at times threatened progress at the site, Silverstein acknowledged, "It's been a very tough time." But, he added, "I've always believed in downtown New York."

In his remarks, New York State Assembly Speaker Sheldon Silver remembered that certain naysayers had warned lower Manhattan "was dead" after the terror attack.

"But Larry, you were right," Silver said. "Never bet against New York."